I am starting with the arduino platform with an adafruit Trinket Pro which is
similiar to the Arduino Pro Mini.
Since switching off/on LEDs is the 'hello world' in the arduino world - I will do
exactly that.
The Trinket Pro is a 3.3V version with 12 MHz and can deliver 20mA max at
each digital pin out. It is using a ATmega328P.
From a local seller I bought at his advice some "low current" LEDs in yellow and
also at his advice - a same amount of resistors - 1200 Ohms each.
Ok.....the LEDs """""""lights up""""""""" and I am sure I will recognize that....at midnight...
But:
The Adafruit SMD-LED at the D13 port is MUCH brighter (the LED is red).
My questions are:
*** What sort of LEDs are the best for using at low current, when I want a good visibility at daylight?
*** My own calculation of the resistor is as follows:
Forward voltage of a yellow LED about 2.0V
3.3V-2.0V=1.3V
1.3V/20mA = 65 Ohm
...what is quite a different value as that the seller of the electronic shop has calculated.
Am I wrong or he?
*** What means "20 mA max"? Is this a totally safe value in the sense of: I can draw 20mA from
a pin for hours without even touching the lifetime of the ATmega chip or do I have to read this as:
"One can draw 20mA for the duration of Dirac pulse only..." ?
Unfortunately I cant find the BOM of the Trinket pro, which would make life a little easier...
Thank you very much in advance for any help!
Best regards,
mcc
65 ohms would be correct, but 68 is a standard value you can buy.
My guess you will not see a difference between 68 and 100, so get some 100, 220, 470, 680 . . .
Just get an assortment
20ma is what is recommended to be safe, safely accommodated by the controller.
20mA continuous current is safe. Btw. There is no such thing as a Dirac pulse in practical electronics because you can't generate infinitely short pulses
Your resistor calculation is correct. 20mA is quite a lot of current for a modern LED though, so the shop clerk probably used 10mA in his calculations.
I wouldn't say your seller was wrong -- if he believed you were just using these LEDs as indicator lights and not trying to illuminate a room or such then 1200 ohms sounds about right. When you start running modern LEDs at 20ma you're going to find that they blind you very quickly.
Chagrin:
I wouldn't say your seller was wrong -- if he believed you were just using these LEDs as indicator lights and not trying to illuminate a room or such then 1200 ohms sounds about right. When you start running modern LEDs at 20ma you're going to find that they blind you very quickly.
The smaller packages like SMD have no almost dispersion, so the die is directly visible and closely lensed. It makes a huge difference, in addition to the phenomenally better efficiencies that they have come up with these days.
Dirac Pulse: I know, that this kind of pulse is a theoretical beast
That's why mentioning it was for humorous purposes only
Otherwise the voltage peak induced (oh, I am not a native english speaker...
I mean "induced"=>"result from induction"="based on induktivity"...ohohoh)
by such a superquick jump of the current would fry at least the ATmega internals
in microseconds....
;)
Science based jokeing is a difficult thing for a person, who is no native speaker
in that language...I simply should shut up...
When I bought the LEDs and resisitors from the seller I said: I want something bright using
a low current (thats why he gave me low current leds). So I think something went wrong in that
conversion...
SMD LEDs: YES! I didn't thought of the nearly missing plastik around the emitting area! Great!
Thank you for mention that! The only thing is: SMD LEDs aren't /THAT/ bread board friendly.
Is there any "with wire"-type of LED which comes close to the visual effects of a SMD LED?
I dont want to brighten a room...It should "only" be visible as a signaling light at daylight outdoors.
Thanks a lot for your help in advance!
Best regards,
mcc
No, I think your seller had the right idea. He gave you high value resistors and very efficient LED's. There is no way he could know exactly how bright you wanted them, without you supplying luminosity specifications.
Almost any LEDs will work for you, provided you connect resistors of the right value. You will have to experiment to find a good value.
What means "20 mA max"? Is this a totally safe value in the sense of: I can draw 20mA from
a pin for hours without even touching the lifetime of the ATmega chip or do I have to read this as:
"One can draw 20mA for the duration of Dirac pulse only..." ?
The forward voltage rating of an LED at the specified current (ie: 2.2V @ 20 mA) means for an indefinate period of time which can be interpreted as the next 200 years continuously . Suffice it to say you won't live long enough to see it fail. There is a very small percentage of leds that fail for no reason but it is rare.
I mean "induced"=>"result from induction"="based on induktivity"...ohohoh)
by such a superquick jump of the current would fry at least the ATmega internals
in microseconds....
I'm really not sure what you mean here. Failure to implement the correct current limiting for leds will almost always lead to device destruction but if the proper resistor is used, it doesn't matter how "quick" the pulse is. We use uS pulses all the time. The shorter the pulse, the less the chance of device damage, so the reference to a "quick pulse" , which in electronics is interpreted to mean a very short rise time , resulting in catastrophic failure of the ATmega chip is nowhere near valid. Your analogy doesn't seem to make any sense, regardless of what language it is in. We understand science, but you haven't presented any and "induction" is not relevant to leds anyway as they are semiconductor devices , not inductive devices. If you have some science to present to support your comment then we would love to see it. Otherwise, like I said, it doesn't seem to make sense. Rise times in the nS or uS would not damage the ATmega chip if the proper current limiting resistors are in place.
Science based jokeing is a difficult thing for a person,